Innovations In Practical Work: Fantastic Plastic
FANTASTIC PLASTIC
The Science Enhancement Programme is a part of Gatsby Technical Education Projects. It is developing
curriculum resources to support effective learning in science, and providing courses and professional
development opportunities for science teachers. This booklet is part of the series ‘Innovations in practical
work’, exploring ways in which low-cost and novel resources can be used in secondary science.
www.sep.org.uk
Gatsby Science Enhancement Programme
Innovations In Practical Work: Fantastic Plastic
First published 2008
by the Gatsby Science Enhancement Programme
Gatsby Technical Education Projects
Allington House (First Floor)
150 Victoria Street
London SW1E 5AE
© 2008 Gatsby Science Enhancement Programme
Authors: Averil Macdonald and Alun Vaughan
Series editor: Richard Boohan
The materials in this booklet may be reproduced for teaching purposes in schools and colleges provided that the
copyright of the Science Enhancement Programme is acknowledged. No copies may be sold for gain.
ISBN 978-1-901351-77-4
ACKNOWLEDGEMENTS
We should like to thank the following for their help in supporting this publication: Joe Keddie for assistance
in developing the original Fantastic Plastic lecture and the Engineering and Physical Sciences Research Council
(EPSRC) for funding the development of the lecture; Peter Stensel of Middlesex University Teaching Resources
for sourcing practical materials; Phil Tetlow of Middlesex University for technical support; all of those who were
involved in giving feedback and in undertaking school trials including Joanne Bolton, Stephen Burrowes, Keith
Strachan and Sally Thurlow; Peter Stensel and Adam Bernard for the photography and illustrations; and Carolyn
Pitkin for providing editorial assistance.
HEALTH AND SAFETY
For practical activities, the Science Enhancement Programme has tried to ensure that the experiments are
healthy and safe to use in schools and colleges, and that any recognised hazards have been indicated together
with appropriate control measures (safety precautions). It is assumed that these experiments will be undertaken
in suitable laboratories or work areas and that good laboratory practices will be observed. Teachers should
consult their employers’ risk assessments for each practical before use, and consider whether any modification
is necessary for the particular circumstances of their own class/school. If necessary, CLEAPSS members can
obtain further advice by contacting the Helpline on 01895 251496 or e-mail science@cleapss.org.uk.
Gatsby Science Enhancement Programme
Innovations In Practical Work: Fantastic Plastic
Contents
Introduction
1
Curriculum links
2
Fantastic plastic in the classroom: ideas and suggestions
5
A history of polymers
Where are polymers used?
What is a polymer?
Thermoplastics and thermosets
How do you make a polymer?
The properties of rubber
Other properties of polymers
Polymer properties related to their uses
Polymers and the environment
Designer polymers – examples of recent and future developments
Student activities
Teachers’ notes
Activity sheets
Extension activities
Background science
Nature of polymers
Molecular structure and macroscopic properties
The future
25
47
References and further reading
53
Downloading resources from the SEP website
54
Obtaining the practical resources from MUTR
55
Gatsby Science Enhancement Programme
Innovations In Practical Work: Fantastic Plastic
What’s in this booklet?
Fantastic plastic in
the classroom
Curriculum links
An ‘illustrated overview’
of the topic with ideas
for practical activities
References and
further reading
Student activities
Photocopiable activity
sheets for students with
detailed teachers’ notes
Background science
Further Resources
DOWNLOADING
DIGITAL RESOURCES
FROM THE SEP WEBSITE
ORDERING
PRACTICAL RESOURCES
www.sep.org.uk
www.mutr.co.uk
See pages 54-55 for further details
Gatsby Science Enhancement Programme
FROM MUTR
Innovations In Practical Work: Fantastic Plastic
INTRODUCTION
Plastics – or polymers as they are more correctly
called – look set to be the material of the future.
Think of any product and it is likely that it
consists, at least in some part, of a polymer: a car,
food packaging, building materials, toys, clothing,
and even food which contains polymers in the
form of sugars and proteins. Every person in the
world is created from DNA, a well-known long
chain molecule – also a polymer.
As we progress through the 21st century, newly
developed polymers will play an increasing role in
our lives – smart polymers in medicine, light-emitting
polymers in entertainment, electrolytic polymers in the
hydrogen fuel cells that are likely to power our cars
and our homes.
The emphasis of this booklet is on providing teachers
with an overview of polymers as they are today,
together with ideas to demonstrate some of their more
unusual properties and the vast range of uses to which
they can be put. The activities are designed to support
the requirements of GCSE science specifications, but
could be adapted for KS3 work or for older students,
including those studying A level technology courses.
A range of low-cost practical resources to support the
activities are available for purchase from Middlesex
University Teaching Resources (MUTR).
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CURRICULUM LINKS
The following table shows the main relevant sections for each of the current GCSE science specifications.
2
GCSE specification
Sections particularly related to polymers
AQA
Unit Chemistry 1b
Oils, Earth and Atmosphere
Alkenes can be used to make polymers such as poly(ethene) and poly(propene). In these
reactions, many small molecules (monomers) join together to form very large molecules
(polymers) (12.4).
Polymers have properties that depend on what they are made from and the conditions
under which they are made. For example, slime with different viscosities can be made from
poly(ethenol) (12.4).
Polymers have many useful applications and new uses are being developed, for example:
new packaging materials, waterproof coatings for fabrics, dental polymers, wound dressings,
hydrogels, smart materials, including shape memory polymers (12.4).
Many polymers are not biodegradable, so they are not broken down by microorganisms and
this can lead to problems with waste disposal (12.4).
Edexcel
Module 5 Synthesis
Recall that polymers are large molecules which can be formed by a combination of many
smaller molecules (C2 5.14).
Draw a section of an addition polymer given the monomer and vice versa (C2 5.15).
Explain the similarities and differences in properties between thermosetting and
thermoplastics in terms of their structure (C2 5.16).
Understand that chemists use information about known reactions to make new chemicals
and predict the products of a reaction given the reactants and products of similar reactions
(C2 5.17).
Explain how addition polymers are formed from unsaturated monomers (C2 5.18).
Predict uses of polymers given appropriate information about their properties (C2 5.19).
Explain how the properties of a plastic can be altered, depending on the starting materials,
conditions of reaction, and additives (limited to plasticisers, preservatives and cross linking),
and relate this to their structure and bonding (C2 5.20).
Discuss the problems of disposing of some plastics, including non-biodegradability and
breakdown to toxic products (C2 5.21).
OCR 21st Century
C2 Material Choices
Interpret information about how solid materials can differ with respect to properties such as:
melting points; strength (in tension or compression); stiffness; hardness; density (C2.1, 1).
Relate properties to the uses of materials such as plastics, rubbers and fibres (C2.1, 2).
Relate the effectiveness and durability of a product to the materials used to make it (C2.1, 3).
Understand that some small molecules can join together to make very long molecules called
polymers and that the process is called polymerisation (C2.2, 9).
Understand that by using polymerisation, a wide range of materials may be produced (C2.2,
10).
Recall an example of a material that has replaced an older material because of its superior
properties (C2.2, 11).
Understand how modifications in polymers produce changes to their properties to include
modifications such as: increased chain length; cross-linking; the use of plasticizers; increased
crystallinity (C2.3, 3).
C3 Food Matters
Recall that many chemicals in living things are natural polymers (limited to carbohydrates and
proteins) (C3.1, 1).
Gatsby Science Enhancement Programme
Innovations In Practical Work: Fantastic Plastic
GCSE specification
Sections particularly related to polymers
OCR Gateway
Module C1 Carbon Chemistry
Item C1e: Making polymers
Construct the formula of an addition polymer given the displayed formula of its monomer.
Construct the formula of a monomer given the displayed formula of an addition polymer.
Describe polymerisation as a process in which many monomer molecules react together to
give a polymer which requires high pressure and a catalyst.
Explain that addition polymerisation involves the reaction of many unsaturated monomer
molecules (alkenes) to form a saturated polymer.
Module C1 Carbon Chemistry
Item C1f: Designer polymers
Suggest the properties a polymer (plastic) should have in order to be used for a particular
purpose.
Explain why a polymer (plastic) is suitable for a particular use given the properties of the
polymer.
Describe that the atoms in plastics are held together by strong covalent bonds.
Relate the properties of plastics to simple models of their structure:
• Plastics that have weak intermolecular forces between polymer molecules have low
melting points and can be stretched easily as the polymer molecules can slide over one
another;
• Plastics that have strong forces between the polymer molecules (covalent bonds or crosslinking bridges) have high melting points, cannot be stretched and are rigid.
Explain why chemists are developing addition polymers that are biodegradable.
Explain some of the environmental and economic issues related to the use of polymers:
• landfill sites get filled quickly wasting valuable land;
• burning waste plastics makes toxic gases;
• disposal by burning or land-fill sites wastes a valuable resource;
• difficult to sort out different polymers so recycling is difficult.
WJEC
Additional Science Chemistry
Know how large saturated hydrocarbons (alkanes) can be cracked by heating in the presence
of a catalyst to form smaller more useful hydrocarbon molecules including monomers
(alkenes) for making polymers (6c).
Know the process of addition polymerisation of ethene to produce polythene (6d).
Be able to draw the repeating unit for the addition polymers, poly(ethene),
poly(tetrafluoroethene) (PTFE) and polyvinyl chloride (PVC) (6e).
Relate the uses of polythene, PVC and PTFE to their properties (6f).
Relate the uses of thermoplastics and thermosets to their properties and structure (6g).
Use given data to compare the properties and uses of polymers and traditional materials (6h).
Evaluate the social, economic and environmental impact of the widespread use of plastic
products, for individuals, communities and the environment. (6i).
Know that the term smart material has been given to a range of modern materials whose
properties change with a change in the surroundings, for example:
• shape memory polymer, which has the ability to regain its original shape when heated;
potential uses include biodegradable surgical sutures and self repairing car bodies;
• polymer gels, such as hydrogels, have a cross linked polymer structure inflated with a
solvent, such as water; intended uses include artificial muscles, robot actuators, absorbers
of toxic chemicals, etc. (7a).
Use given data to evaluate the social, economic and environmental impact of the use of
smart materials (7b).
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Innovations In Practical Work: Fantastic Plastic
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Innovations In Practical Work: Fantastic Plastic
FANTASTIC PLASTIC IN THE CLASSROOM:
IDEAS AND SUGGESTIONS
Polymers are all around us. This is probably why we take them for granted and
assume that the properties of polymers are typically fairly mundane and certainly
are unlikely to develop. We couldn’t be more wrong! Not only do presentday polymers exhibit a huge range of properties, but new polymers are being
developed all the time with ever more impressive properties.
This section gives an ‘illustrated overview’ of how practical work on plastics can
be undertaken in the classroom. In the margin, references are given to student
activities – these resources and the accompanying teachers’ notes can be found
later in this booklet.
A HISTORY OF POLYMERS
The earliest polymers to be exploited by mankind were natural materials: wood
for structural members, cotton for clothing, and so on. However, polymers have
also been extracted from biological systems and their properties exploited for
thousands of years. Locust bean gum is a member of the galactomannan family
that can be extracted from the fruit of the carob tree, which has long been
cultivated widely around the Mediterranean. Indeed, it was used in ancient Egypt
as an adhesive in mummification.
Examples of natural
materials that were the
earliest polymers to be
used by humans: wood
(left) and cotton (right).
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It is only the last 150 years or so that synthetic and semi-synthetic materials have
been deliberately developed for commercial exploitation. The first commercial
materials to be developed were derived from natural polymers, notably, natural
rubber. Around 1840, it was discovered, independently by Hancock in the UK and
Goodyear in the US, that reacting rubber with sulphur reduced its tackiness and
improved its elastic properties. This was a cross-linking reaction that resulted in
the formation of a molecular network. At around the same time, it was discovered
that by reacting cellulose with nitric acid, it was possible to produce a material
with numerous useful properties – cellulose nitrate. This material could be spun
into fibres to form ‘artificial silk’ and its main application was as gun cotton.
Cellulose nitrate is a low order explosive! However, for a short time, it was also
used in billiard balls. The mild explosions that occasionally resulted from violent
cannoning of the billiard balls were an undesirable side-effect.
One of the first commercial
polymers developed was
rubber in the mid-19th
century and used for solid
tyres on the earliest bicycles
(left); the first truly synthetic
polymer was Bakelite in
the early 20th century and
was used for a number of
domestic items such as this
camera (right).
At the Great Exhibition in London in 1851 a medal for quality was awarded to
Alexander Parkes for items made from a mixture of cellulose nitrate and castor
oil as a plasticiser to make it more flexible. Based upon this John Hyatt created
celluloid by using camphor instead of castor oil as the plasticiser added to
cellulose nitrate to make it even softer. Probably the most influential application
of this polymer resulted from its adoption in 1899 by Eastman Kodak as the base
for its film products. However, it is extremely inflammable and degrades relatively
rapidly releasing nitric acid, so that most early films have now been lost.
The first truly synthetic polymer was developed by Leo Baekeland around 1910,
who reacted phenol and formaldehyde under controlled conditions to form the
family of materials we know as Bakelite.
Things moved quickly and 1933 saw the birth of polyethylene by Gibson and
Fawcett at ICI in the UK (Polythene is the Du Pont trade name), followed by
Perspex (poly(methyl methacrylate) or PMMA), polystyrene, and poly(vinyl chloride)
or PVC. Carothers’ work on polyamides around the same time led to the discovery
of Nylon.
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Developments on polymers
in the 1930s led to many
of the familiar plastics we
use today: polyethylene
milk bottle, polystyrene
fast-food container, PVC
pipe and nylon rope.
Although both the polyethylene and polyamide families are long chain molecules,
they are formed by fundamentally different processes. Polyethylene requires
a step-growth process whereas Carothers’ initial work resulted in division of
polymerisation reactions into two distinct types: condensation and addition
reactions. For more information on the chemistry of these processes and the
different physical properties they confer on the polymer, see the Background
science section on page 48.
Despite all this technological advance, up until about 1930, there was no
consensus about the molecular nature of this class of material. While it was
accepted that they contained large structures, it was initially assumed that that
these took the form of colloidal aggregates of smaller molecules, held together
by secondary forces. However, the credit for establishing the real science of
polymers can be given to Hermann Staudinger who, in the 1930’s came up with
the revolutionary idea that polymers were composed of long chains of smaller
building blocks held together by covalent bonds making macromolecules. He was
awarded the Nobel Prize in 1953.
WHERE ARE POLYMERS USED?
Polymeric materials now encompass a vast range of biological and synthetic
materials that are used in a wide range of applications. The market for polymers
involves many large commercial organisations that compete with one another in
different market sectors. Thus, polymer nomenclature involves chemistry, from
various eras, plus commercial trade names, all of which are used synonymously.
The vast majority of the items we call ‘plastic’ are made from one of five ‘families’:
1. PE: polyethylene (low density = LDPE, high density = HDPE), e.g. carrier bags.
2. PP: polypropylene, e.g. drinks cups, kettles, shampoo bottles
3. PS: polystyrene and expanded polystyrene (EPS), e.g. insulating cups, packaging
4. PVC: poly(vinyl chloride), e.g. window frames, insulation cables for domestic wiring.
5. PET: poly(ethylene terephthalate), e.g. transparent water bottles.
The precise polymer chosen for any application depends upon the combination
of properties required, such as its glass transition temperature (below which the
polymer becomes brittle), its melting point, its toughness, its aging properties,
whether it absorbs the chemicals it is to contain, how easily it can be moulded/
shaped and the cost.
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The name of a polymer is derived from the monomer, for example propylene or
ethylene terephthalate. If the monomer is a simple molecule such as ‘propylene’
then the name of the polymer is ‘polypropylene’ – with no brackets. If it is
a combined molecule such as ‘ethylene terephthalate’ then the polymer is
‘poly(ethylene terephthalate)’ – with brackets.
Examples of commonly used trade names are NylonTM (polyamide), PolytheneTM
(polyethylene), KevlarTM (poly(paraphenylene terephthalamide), TeflonTM
(polytetrafluoroethylene or PTFE) and PerspexTM (poly(methyl methacrylate) or
PMMA).
Throughout this booklet reference will be made to a whole host of everyday
plastic items we take for granted that can easily be used to demonstrate the
range of polymer properties and how they can be exploited to create useful
products including:
• Drinks can retainers (elasticity)
• Cosmetics / hair gels (gel consistency for spreading)
• Chewing gum (melts at body temperature)
• RFID tags (conducts electricity cheaply)
• Nappies (absorbs fluids)
• Solvent absorbing hydrogels (absorbent)
• Hot glue guns (thermoplastic)
• Post-it notes (sticky at room temperature)
• Police riot shields (tough yet lightweight)
• Medical e.g. sutures, valves, joints, possibly organs (inert in the body)
• Intelligent packaging (changes colour when in contact with deteriorating food)
• Heat insulation for houses, packaging (low thermal conductivity)
• Polystyrene shelters for disaster victims (lightweight and robust).
Polymers are now
used extensively in a
wide variety of familiar
everyday items.
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Despite the huge range of items made from plastic – the worldwide production of
polymers is estimated at 245 million tonnes per year – it still consumes only 4%
of the world production of oil.
Although plastic is seen by many as a modern evil, without plastic packaging the
alternatives would weigh four times as much, increase transport costs with the
greater amount of fuel needed to transport everything from household goods
to food, cause the emission of twice as much greenhouse gas, and create more
waste to be disposed of.
Plastic packaging in the food industry increases shelf life and reduces food waste
– it is estimated that the plastic around a cucumber typically increases its life from
3 days to 14 days. And it’s worth remembering that the amount of carbon dioxide
used to produce meat is 100 times as much as the plastic film used to package it
and prevent it going off too quickly.
Plastics as insulators are major components in our attempts to reduce energy
consumption in the home in our fight against global warming.
Globally 50% of polymers are recovered when disposed of, either through
recycling or through energy recovery. Some countries recover 100% of disposed
polymers so they have totally eliminated plastics going to landfill – though the UK
is one of the worst, recovering only 20%.
It is possible to recycle most of the commonly used polymers, for example, PET
from plastic drinks bottles is often recycled as fibres in the ‘stuffing’ of jackets.
Unfortunately the process for separating the various polymers before recycling is
complex and expensive. Typically the polymers have to be ground down into pellets
and then submerged in liquids in order to separate them by density. In most cases
the process is, at present, economically unviable. Even if the process becomes more
financially attractive PVC is unlikely to be recycled as it is usually filled with various
colorants and fillers which would have to be removed before re-use.
WHAT IS A POLYMER?
Any long chain molecule is a polymer. Wood is a polymer too – cellulose is a
polymer. In fact the human body is composed of long chain molecules – DNA,
proteins – and so can quite reasonably be described as a self-replicating polymer.
Polymers are constructed by polymerizing or linking many ‘monomers’. The term
polymer is derived from the Greek poly, meaning many, and meros, meaning
part and, as such, refers to materials in which the constituent molecules can be
thought of as being made up of a sequence of many simpler units.
The majority of polymers are derived from oil products, through cracking to
produce alkanes and alkenes, which then through polymerization form wellknown polymers. Polyethylene is formed from the monomer ethene, and is used
typically in carrier bags. Polypropylene, formed from the monomer propene, is
a tougher material used for example in making crates. However, its low glass
transition temperature means that if left out during cold weather it is likely to
become brittle.
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See Activity A1:
Can you cut a liquid?
(page 34)
Classroom activities
An effective demonstration of evidence that polymers are composed of long
chain molecules is to contrast what happens when you try to cut a liquid such as
water (composed of separate molecules) with cutting a polymeric solution – of
poly(ethylene oxide) or ‘polyox’. This should be done as a teacher demonstration.
Students can see the effect of the long chain nature of the polymer: when the
solution is cut and the stress is suddenly released, the elasticity of the solution is
dramatically demonstrated, as the liquid jumps back into the beaker.
Long polymer molecules intertwine or
‘entangle’ with each other, just like spaghetti
on a plate. These physical entanglements
allow polymer solutions to bear some
mechanical stress. The molecules are usually
coiled up, but when stress is imposed on
them they can straighten out somewhat.
When the stress is released, they coil up
again. This coiling process gives a polymer its
elasticity.
A solution of polyox
can be cut with a pair
of scissors as it is being
poured.
All long polymer molecules in a sufficiently
concentrated solution will have physical
entanglements that lead to viscoelastic
behaviour. However, there is an additional
factor at work in polyox. Water molecules
can bridge two adjacent polymer molecules
by forming hydrogen bonds. These bridges
or ‘cross-links’ create a strong network of
molecules. A three-dimensional network of
molecules in a solution is referred to as a gel
(see the section below ‘How do you make a
polymer?’ for more about cross-linking).
This demonstration works because of the molecular chain length of the polyox.
Greater molecular weight means longer chains. Their entanglement is greater,
resulting in a more viscous liquid. The demonstration has to be done with
a relatively high molecular weight polyox (i.e. long chain molecules). Lower
molecular weight liquids are less viscous as the molecules are too short to
entangle. The way that the molecular chains flow and tangle is described as
‘reptation’ from ‘reptare’ (to crawl in Latin) as the molecules are visualised as
flowing and tangling like snakes.
Another simple demonstration of the long chain nature of polymers is to take
standard plastic ‘can retainers’ (transparent plastic loops used to keep four drinks
cans together for sale in supermarkets). Pulling on the side of one of the loops
will gradually cause the plastic to stretch and also to separate into strands which
can be seen very effectively if placed on an overhead projector and observed on
the screen.
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THERMOPLASTICS AND THERMOSETS
The two main classes of synthetic polymers are themoplastics, which can be
moulded since they are able to flow when heated but harden on cooling, and
thermosets, which do not flow on heating – in fact they will decompose before
there is any change in their mouldability. The difference lies in the way the
molecules are bonded.
Thermoplastics, such as polystyrene (often used to make vending cups, the inserts
in food packaging or the transparent packaging for Easter Eggs or toys), are
formed of linear or branched molecules which are able to move past each other
on heating.
Themosets have been cured through a chemical reaction, to form a well-defined
huge molecular network through ‘cross-linking’ (see pages 13-14 for more detail
on this). An example is epoxy resin which is liquid when uncured but the addition
of the ‘hardener’ results in a molecular network being formed creating a hard
material – a reaction which cannot be reversed without further chemical reactions.
N.B. For some thermosets the curing process (chemical reaction) can be induced
by heating, but it is not a necessary element of the process, i.e. thermoset does
not mean set through heat!
See Activity A2:
Food wrappers party tricks
(page 35)
Classroom activities
A high-powered hair dryer or hot air gun can be used in a teacher demonstration
to shown the differences between themoplastics and thermosets. Most shaped
inserts for food packaging have been formed from thermoplastic materials. The
initial flat sheet is heated and then moulded by vacuum compressing over a
shaped mould then allowed to cool. Reheating the formed shape using a highpowered hair dryer or hot air gun allows it to revert to its original shape of an
(almost) flat sheet. This also works with the transparent plastic front wrappers of
Easter Eggs and toys as well as drinks cups.
All of the packaging
shown here (left) is made
of polystyrene; a plastic
vending cup is also made of
polystyrene and will flatten
when heated.
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A hot glue gun, often used in design and technology classes, is another good
example of a thermoplastic. Anyone using a hot glue gun will be aware of the
potentially annoying fine fibres of glue that are drawn. This, again, is evidence of
the polymer being composed of long molecules tangled up.
A hot glue gun (left) heats
the polymer used for the
glue creating fine fibres
(right) showing that this is a
thermoplastic.
One of the practical activities described later in this booklet (Activity A5) involves
the use of a polymer called polymorph – also a thermoplastic. In contrast, rubber
(used in Activities A4, A5, A6 and A7) is a thermoset.
See Activity A3
What’s that polymer?
(page 36)
Polymer properties vary according to the way the polymer is processed and the
fillers added, and students can explore this by looking at a number of items
made from polystyrene with different properties that suit them for their job. The
properties also depend on the molecular chain length of the polymer: longer
chain lengths will produce a tougher more brittle material, and students can
observe how the properties vary across a range of materials that have polymers
with different chain lengths.
A variety of
items made from
polystyrene processed
in different ways.
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HOW DO YOU MAKE A POLYMER?
In a polymer, long-chain molecules are made by joining together the same
molecular groups (or repeat units) to make a long chain. For example, the polyox
or poly(ethylene oxide) described earlier in the section ‘What is a polymer?’ is
produced from a reaction of ethylene glycol (HO–CH2–CH2–OH). Initially, two
monomers join together with the elimination of a water molecule:
HO–CH2–CH2–OH + HO–CH2–CH2–OH => HO–CH2–CH2–O–CH2–CH2–OH + H2O
The addition of further monomers produces a long chain:
HO–CH2–CH2–[–O–CH2–CH2–]n–OH
Note that the repeat unit for poly(ethylene oxide) is –[–O–CH2–CH2–]-. The scheme
shows the complete structure of the polymer molecule – n represents the number
of times the unit is repeated, and can typically be several thousand. Notice that
there is an oxygen atom at every third atom along the chain. This oxygen readily
forms hydrogen bonds with water, giving the polymer a very high solubility in water.
The process of creating a polymer from its monomers is polymerisation. The
example above is a condensation polymerisation, so called because a simple
molecule (in this case water) is eliminated during the reaction. Addition
polymerization occurs when unsaturated monomers combine together without the
elimination of simple molecules. Polyethylene is an example of an addition polymer.
Step-growth polymerization happens when any pair of monomers might react and
begin to form short chains, which, in turn, can react. Nylon is step-growth. Making
nylon is an interesting and well-known practical demonstration and details for this
can readily be found from other sources (see References and further reading on
page 53).
Cross-linking is a means by which a polymer’s molecular structure can be changed
in order to give the polymer different properties. Cross-linking results in the
creation of networks of molecules linked to each other by covalent bonds. The
degree of cross-linking determines the final properties of the material. Slime is
a good example of a partially cross-linked material (details of how to make and
test slime can be found in the SEP book ‘Go with the Flow’ – see References and
further reading on page 53), as are cosmetic materials such as hair gel. Chewing
gum and ‘potty putty’ are also partially cross-linked polymers. The property
that people enjoy with potty putty (and chewing gum) is the fact it flows easily
when pulled gently or left on the table, but will break when pulled hard. This
demonstrates the fact that the molecules are able to flow past each other slowly
when subjected to a gentle force, but are not able to flow quickly and will ‘break’
when subjected to a sudden force.
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Some examples of
materials that contain
cross-linked polymers.
Once cross-linked, the material cannot easily be changed as the cross-linking is
irreversible. Rubber in car tyres was traditionally cross-linked by vulcanization (a
chemical process involving exposing the rubber to sulphur). As this cross-linking
cannot be reversed, there are considerable challenges in disposing of car tyres at
the end of their useful life since they cannot be degraded. Some use is made of
them mixed with tarmac to coat roads with a sound-absorbing surface but we are
still faced with huge mountains of car tyres.
The name ‘rubber’ is widely used to describe all synthetic elastomers. The name is
believed to have been given to this material by Joseph Priestly in England in 1770,
who discovered that dried rubber latex rubbed out pencil marks.
Natural rubber latex is the sap of a rubber tree Hevea brasiliensis. It is an elastic
hydrocarbon polymer (i.e. it consists mainly of carbon and hydrogen). Essentially
it is a polymer of isoprene units creating polyisoprene with a molecular weight of
100,000 to 1,000,000. Synthetic rubber can be made by deliberate polymerisation
of a variety of monomers to produce polymers.
See Activity A4:
Making a rubber ball
(page 37)
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Classroom activities
Students can make their own solid rubber from liquid rubber latex. If rubber latex
is mixed with a weak acid such a lemon juice or vinegar it will cross-link. By simply
adding the acid to a small quantity of liquid rubber latex and stirring, a solid
lump of rubber will be produced. The weak acid makes the rubber cross-link: this
means that covalent bonds form linking one polymer chain to another, causing
the liquid rubber latex to become solid. Thus the solid form, being composed of a
huge molecular network created by cross-linking, is an example of a thermoset.
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Rubber can be made from
liquid rubber latex and a
weak acid such as lemon
juice (1). After mixing
the acid with the latex,
solid rubber is formed
(2), which can then be
dropped in water and
moulded into a ball shape
(3), before drying it on a
cloth (4).
See Extension X1:
Making your own rubber
eraser (page 45)
1
2
3
4
Having made the rubber ball, students can carry out an investigation into its
elasticity by seeing how far it will bounce (see the next section The properties of
rubber). Using a similar procedure, students can also make their own rubber eraser,
and test how good it is at removing pencil marks from paper.
THE PROPERTIES OF RUBBER
Natural rubber is an elastomer. The word elastomer comes from two terms, elastic
(which describes the ability of a material to return to its original shape when a load
is removed) and mer (from polymer).
When a rubber ball hits the floor it feels a strong force (or stress). The stress causes
the rubber to deform on impact (it flattens). It is the covalent cross-linkages that
ensure that the rubber will return to its original shape when the stress is removed so
that the ball bounces.
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Classroom activities
Students can use a rubber ball
that they have made themselves
(see previous section) to compare
the ‘bounciness’ of rubber with
another polymer called ‘polymorph’
which can also be easily moulded
into the shape of a ball. They can
drop the rubber ball from a given
height and measure the bounce
height, and can repeat this across a
range of different heights.
The elasticity of a rubber
ball can be tested by
comparing the height from
which it is dropped with its
bounce height.
See Activity A5:
Testing bounce
(page 38)
Polymorph is a thermoplastic material that softens and can be moulded at a
relatively low temperature. It is supplied in the form of granules – when these are
added to water at about 60 – 65 oC. The granules soften and stick together in a
clump which can then be moulded into a shape by hand. Care needs to be taken
to avoid scalding by ensuring that the water is not too hot (it is advisable to use
a thermometer to monitor the temperature) and by drying the polymorph before
moulding by hand. Once it has cooled, the ‘bounciness’ of the polymorph ball can
be compared with that of the rubber ball.
Making polymorph: the
granules soften when put
into warm water (1), and
can then be removed as a
clump (2) and moulded by
hand (3).
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See Activity A6:
Make your own rubber band
(page 40)
The elasticity of rubber can also be tested by making a rubber band and then
measuring its extension when a force is applied. To make the rubber band, students
dip the end of a pencil (or wider item) into rubber, then into water and finally into
lemon juice before gently rolling it off the end of the pencil to form a ring.
Making a rubber band:
a pencil is dipped into
rubber latex (1). After
putting into weak acid to
make the rubber cross-link,
and then into water, the
rubber can be rolled off
the pencil (2) to form a
ring (3).
2
1
Measuring the extension
of the rubber band
against force.
3
Students can now test the
elasticity of the rubber
band by adding slotted
masses and measuring
how far it stretches. They
can find out whether the
rubber band obeys Hooke’s
Law, i.e. whether its
extension is proportional to
the applied force.
Heating a stretched rubber strip makes it contract. This seems odd as most
materials expand when you heat them. Whether a material expands or contracts
when it is heated depends upon the material’s entropy. The entropy of a material
is a measure of the organization or orderliness of the molecules. For molecules
that are arranged in an orderly way, the material has a low entropy and when the
molecules are disordered, the entropy is high.
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When a material is heated, its entropy increases because its molecules move about
more and so become more disordered. Rubber contains very long chain molecules.
When rubber is heated, sections of the molecules move about more energetically
and pull on neighbouring parts. As the middle of the molecule moves more it
tends to pull on the nodes of the molecular network (the link points), which then
move closer together causing the rubber strip to get shorter.
This can easily be demonstrated by hanging approximately 1kg from a thick
elastic band and then heating the band using a hot air gun. The length of the
stretched band should be noted initially. It will shrink by around 1 cm quite
quickly when heated.
A stretched rubber band
contracts when heated.
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See Activity A7:
Making and testing foam
rubber (page 42)
If rubber is cross-linked while carbon dioxide is bubbling through it, the rubber
forms a foam with the gas bubbles set into the rubber. These make foam rubber
more compressible than the solid form. Foam rubber is easily made by using
a weak acid to cross -link the rubber as for making a rubber ball, but adding
sodium bicarbonate to the mixture. This reacts with the acid to produce carbon
dioxide. By creating a foam rubber pad and a flat rubber pad, a comparison
can be made between the two to see the effect on the compressibility of
incorporating the gas.
Foam rubber can be
made from rubber latex,
citric acid and sodium
bicarbonate (1); when
mixed together bubbles
of carbon dioxide are
produced (2). The
properties of the foam
rubber produced can be
compared to a similar
piece of rubber which
has been produced
without foaming (3).
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OTHER PROPERTIES OF POLYMERS
Polymers can be designed to have pretty much any property you wish including
flexibility, elasticity, toughness, stickiness, temperature sensitivity, light sensitivity,
chemical sensitivity, electrical insulation, electrical conductivity, thermal insulation,
shape memory, selective absorption, melting temperature, glass transition
temperature, solubility in water or other solvents, and degradability.
Electrical insulators Polymers are well-known as electrical insulators. Their
lack of mobile charge carriers ensures that no charge can flow under normal
circumstance – though even the best electrical insulators can ‘break down’ under
extremely high fields. The insulators surrounding underground electrical cables
have to be very rigorously tested for breakdown strength as, should the cable
insulation break down, it would blow a huge hole in the ground as the high
voltage shorted to earth.
A balloon can be
electrically charged by
friction because it is a
good electrical insulator.
One simple demonstration of the electrical
insulation properties of polymers is charging
a polymer by friction. Any polymer, such as
a balloon, can be changed by friction (in the
case of a balloon, by rubbing on a woollen
jumper). As the balloon is an insulator, the
charge does not flow away but remains on
the balloon’s surface. Putting the balloon
against a neutral wall will result in it sticking
as the charged balloon is attracted to the
induced charges in the wall.
Electrical conductors Polymers are now being developed that can conduct
electricity. One such polymer is polyaniline; although it was discovered in 1862, it
has only just been recognised as a conducting polymer, as no-one had previously
checked its conductivity! Conducting polymers are leading to RFID tags (Radio
Frequency Identification tags) being applied to a whole host of products for a
range of purposes. Instead of barcodes on food wrappers, there will be RFID tags
which will carry the same information that a barcode carries today (or perhaps
more). This means that checkouts in supermarkets will not need lasers to read the
barcode but will read the RFID tags electronically.
Smart polymers Polymers are now being developed with ‘smart’ properties.
Some are able to change colour with temperature. These are used for simple
thermometers which are applied to a child’s head to determine whether they have
a fever, or can be built into mugs or even clothes so that they change colour with
the temperature of their contents. Other smart polymers can be moulded when
warm (thermoplastic) but will revert to a predetermined shape when reheated. An
example of this is ‘shape memory polymer’ (see Obtaining the practical resources
on page 55).
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A strip of shape memory
polymer composite has
been warmed, twisted
into a new shape and
left to cool (left). When
warmed again by dipping
in hot water, it reverts to
its original shape (right).
See Activity X2:
Why do plastic bags sound
different? (page 46)
Classroom activities
Polyethylene has a range of properties, depending on whether it is high density
or low density. These properties can be investigated and include sound, density,
and melting point. The best material to demonstrate this is ordinary plastic
carrier bags. Students can try to classify plastic bags by whether they are ‘crackly
sounding’ or ‘quiet’ and then to compare the density of the two groups. Bags
made from high density polyethylene are more ‘crackly’ than those from low
density, as it is more crystalline (see the Background science section on page 48
for more information about the forms of polyethylene).
POLYMER PROPERTIES RELATED TO THEIR USES
Some polymers are hydrophobic or ‘water-fearing’, which means that water
will not spread on their surface and they will not dissolve in water. A common
example is polytetrafluoroethylene or PTFE, known commercially as TeflonTM. This
consists of long chains of carbon with two fluorine atoms bonded to each carbon
atom. Replacing the fluorine atoms with hydrogen atoms creates polyethylene,
which is likewise hydrophobic. If PTFE is drawn out into fibres, the result is a
material with networks and voids such that water vapour can pass through but
water droplets cannot. This is now exploited as a breathable fabric under the
trade name of Gortex.
Polymers that absorb and dissolve in water are known as hydrophilic or ‘waterloving’. Most hydrophilic polymers contain –OH (hydroxyl) groups along the length
of the molecule. These groups form hydrogen bonds with water and are attracted
to polar molecules.
Sodium polyacrylate is a hydrophilic polymer, and is used in a material known as a
‘hydrogel’. The electric charge on the polymer enables it to hold even more water
than neutral hydrophilic polymers. When the sodium counter ion is dissolved in
water it is surrounded by several water molecules, and other water molecules
are able to create hydrogen bonds with the polymer. There is still an attraction
between the positive sodium ions and the negative polymer, and this attraction
can be thought to be holding the whole structure together.
There is another kind of polymer material that can absorb not water but oil.
Rather than being hydrophilic, this type of material is ‘lyophilic’ or ‘oil-loving’. The
material is a polymer gel that can absorb up to half of its own weight in oil. The
gel is spread on the oil spill and forms a thick ‘carpet’ which is then rolled up and
taken away. The oil can later be removed by reversing the absorption process. Two
additional uses of this material may be in transporting drugs around the body and
as a replacement for the silicone oil used in present-day breast implants.
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See Activity A8:
What is a hydrogel?
(page 44)
Classroom activities
Students can test the claim that a
given mass of a hydrogel is able to
absorb its own mass of water. (In
fact it can absorb several times its
own mass.) Hydrogels are used in
disposable nappies, and students
could compare the absorbancy of
nappies from different suppliers.
Hydrogel is put into a petri
dish (1) and water added
until it can absorb no more
(2). Weighing before and
after shows that it can
absorb several time its
own mass.
1
2
POLYMERS AND THE ENVIRONMENT
Increasing attention is being paid to degradable and biodegradable polymers.
A biodegradable polymer is, technically, one that degrades due to naturally
occurring organisms. Degradable is more correctly applied to polymers that break
down often due to environmental conditions. Although biodegradable polymers
were first attempted in 1960’s by adding starch to polyethylene (a constituent
of many bags and packaging materials), they met with little success. Now
polyethylene can be made to degrade through the inclusion of additives which
result in chain scission under the action of light, heat or mechanical stresses. This
is the cause of carrier bags becoming fragile and, finally, flakey. Alternatively it can
be co-polymerised with starch. The starch will be consumed by microorganisms
leaving voids in the structure and giving it a much larger surface area. This means
it is more permeable to oxygen, which will ultimately weaken it. However the
material still requires additives to make it degrade completely.
The final products of degradation are water and carbon dioxide plus a small
amount of residue. Thus degradable or biodegradable polymers should not be
seen as a good solution to the litter problem as they are still producing harmful
greenhouse gases as they degrade.
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Polylactide is naturally biodegradable and is derived from renewable sources such
as maize. It also consumes carbon dioxide during production and is completely
recyclable and compostable. It can be used in food packaging and in the medical
industry. However it is much more costly than non-degradable polymers, and
tends to be more brittle and less flexible, so companies tend to prefer more
established polymers at the moment. However if cost implications can be
overcome and the mechanical properties can be improved this would seem an
excellent way forward.
DESIGNER POLYMERS – EXAMPLES OF RECENT AND FUTURE DEVELOPMENTS
Synthetic polymeric materials have only been used for the last 100 years or so
but have already had a massive technological impact. Indeed, take a look around
– what is it that you can see that is not a polymer? Polymer fibres are used in
many clothing applications, polymers as the basis of paints and coatings, polymer
adhesives hold together many artefacts, and the casings for the vast majority
of electrical items are made from plastic. In the vast majority of these instances
though, the polymers are serving a rather mundane role: they are important
in that they are cheap and adaptable and, thus far, that has been their major
advantage.
However, this situation is changing. In the future, plastics won’t just be the
passive boxes that hold all the clever stuff in your computer display – they will be
doing all the clever internal stuff as well. The rate of change in this area since the
late 1970s has been enormous, and it is not possible at present to be certain of
the future or to provide a comprehensive overview of current developments. Here
though are some examples to illustrate how polymers are evolving from worthy
engineering material to exciting stars that will make a whole new generation of
technology possible.
Semiconducting or light-emitting polymers Recent developments in
conducting polymers have led to semiconducting polymers, and light-emitting
polymers (OLEDs). Flat screen televisions measuring only 3 mm in thickness, and
even screens that are flexible and can be rolled up, are being developed using this
technology. There are also proposals to replace all light bulbs with light-emitting
polymers in homes and offices. These will be more efficient than low-energy light
bulbs and will rarely need replacing.
Electro-active polymers (shape memory/actuators) More polymers are being
developed that have an additional property – that of changing shape when an
electric field is applied. The aim is to develop these as replacement muscles,
particularly in prosthetic limbs or hands, so that amputees can control their limbs.
Another polymer, poly(vinylidene fluoride) or PVF2, is a piezoelectric material,
generating a field across it when under pressure (or vice versa). It is being used in
ultrasound transducers, microphones and sonar transducers.
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Electret fibres for dusters and toys Electret fibres are polymeric fibres that
can be woven into products such as carpets, dusters, and even stuffed toys. The
charges on the fibres are unevenly distributed so that different parts of the fibre
will attract small charged particles – such as dust! For a duster, this makes it more
able to pick up and retain dust. For a carpet or a stuffed toy, the fibres trap and
hold dust and allergens such as house dust mite, so that children with asthma are
less likely to be affected by dust in the air.
Medical applications Polymers have a huge, and growing, range of uses in the
medical field. Presently we use replacement plastic joints, veins, skin (for burns
victims), lenses (for cataract sufferers) and heart valves. Slow-release drugs are
impregnated in degradable polymers, while recently a fully functioning bladder
was ‘grown’ from cells impregnated into a polymeric matrix for transplant. The
vision for the future is to be able to grow fully functioning replacement livers and
kidneys to remove the need for live donor organs. There is even the possibility of
artificial blood to supplement real blood during transfusions.
Polystyrene shelters for disaster victims When a natural disaster causes
people to lose their homes, one of the biggest problems is to provide cheap,
quickly erected but robust shelters. A recent development has been to use
expanded polystyrene shelters – these are easy to ship to refugees and simple
to erect, and are robust and warm. Later, the polystyrene frame can be used as
a base onto which concrete is applied to make the shelters longer lasting until
proper housing can be created.
With this range of properties, the potential for new polymers remains
unimaginable. Who knows what new polymers are just around the corner?
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STUDENT ACTIVITIES
Fantastic plastic
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Teachers’ notes
The student materials consist of a series of practical activities intended for use at KS4. The key ideas are:
• Polymers are composed of long chain molecules.
• Polymers can be divided into two groups: thermoplastics which deform when heated due the intermolecular
bonds being relatively weak, and thermosets which do not change when heated due to being formed of a
molecular network of much stronger covalent intermolecular bonds.
• Heating a thermoplastic provides the energy that allows it to change shape or flow.
• Polymers are composed of monomers that are then joined into long chain molecules by a process of
polymerisation.
• Polymers in liquid form can be cross-linked to create a solid by the formation of covalent bonds, creating a huge
molecular network or what is, in effect, one giant molecule.
• Natural rubber is an elastomer. The word elastomer comes from two terms, elastic (which describes the ability of
a material to return to its original shape when a load is removed) and mer (from polymer).
• When a rubber ball hits the floor it feels a strong force (or stress). The stress causes the rubber to deform on
impact (it flattens). Relaxation of the rubber to its natural shape results in thermal energy of the material being
converted to kinetic energy.
• It is the covalent cross-linkages that ensure that the rubber will return to its original shape when the stress is
removed so that the ball bounces.
• Adding gas to a polymer as it cross-links can incorporate bubbles into the structure which gives it the property
of being compressible due to the spaces in the rubber.
• Different polymers have different properties, for example different densities of the same polymer will have
different properties. This is what determines its potential uses.
Visit the SEP website www.sep.org.uk for supporting resources including image files and editable versions of the
student sheets. See page 55 for information on obtaining the practical resources.
An overview of the activities
The activities are:
•
•
•
•
•
•
•
•
Activity A1 Can you cut a liquid?
Activity A2 Food wrappers party tricks
Activity A3 What’s that polymer?
Activity A4 Making a rubber ball
Activity A5 Testing bounce
Activity A6 Make your own rubber band
Activity A7 Making and testing foam rubber
Activity A8 What is a hydrogel?
There are two extension activities:
• Extension X1 Making your own rubber eraser
• Extension X2 Why do plastic bags sound different?
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Notes on the activities
Activity A1: Can you cut a liquid?
This activity is a teacher demonstration that introduces the idea of long chain molecules (see also page 9 of this booklet ‘What is
a polymer?’).
LEARNING OBJECTIVES
Students will:
MAKING UP THE SOLUTION
The solution should be made in advance.
• observe how a polymer solution can be ‘cut’ when
poured
• interpret the behaviour of the polymer in terms of its
composition of long chain molecules.
• 25 cm3 propan-2-ol (or other anhydrous alcohol, e.g.
ethanol)
• 3 – 4 grams polyox [high molecular weight poly(ethylene
oxide)]
• Stirring rod
• 2 x 600 cm3 beakers
• Colouring e.g. food colouring or fluorescein (optional but
fun!).
NOTES
This is a useful demonstration that polymers are made
of long chain molecules. Contrast the behaviour of this
viscoelastic polymer liquid with a more typical liquid
composed of small, individual molecules that can easily
move past each other.
The polyox solution should be made up in advance. It is
not recommended that students make up the solution
themselves – or that they try cutting the liquid. This should
be done as a demonstration by the teacher.
Only try to cut the liquid with the upper beaker held
almost upright (not more than 30o to the vertical) and not
leaning over as gravity will make it continue to pour and
create a mess on the scissors. Pour the liquid from the
side of the beaker (not the lip) to get a reasonable flow,
return the beaker to an almost upright position then cut
liquid sharply around 1 cm below the edge of the beaker.
The liquid will visibly jump back in to the beaker. This takes
practice!
N.B. This polymer is extremely slippery and great care
should be taken when handling beakers if liquid has been
spilled on the outside.
As an additional demonstration of long chain molecules,
you could use drinks cans retainers (the type used to hold
4 cans together). Pull one ‘loop’ slowly by hand and note
that the materials ‘shreds’ into strands. This can be seen
easily by eye or can be placed on an old-fashioned OHP
and projected. The shredding demonstrates the orientation
of the molecules and that polymers are made of long
chain molecules. The ‘strings’ that are drawn from the
glue in a hot glue gun are also evidence of the long chain
nature of polymers, as are the long strings that you can
pull from chewing gum and from potty putty.
Mix 25 cm3 alcohol with 3 – 4 grams polyox in a clean dry
beaker. This is to disperse the powder, not to dissolve it.
Add 350 – 400 cm3 tap water in one quick pour (as quickly
as you can!) and stir until the polyox powder has dissolved.
As you stir, the viscosity of the polymer solution will increase.
Add colouring if required – food colouring or fluorescein.
The gel, once made, can be kept for many months in a
watertight container. It will become thin after a while. Simply
sprinkle some polyox powder onto the surface of the liquid,
shake to disperse and wait 24 hours for it to dissolve and
your liquid will be thick enough to cut again.
HEALTH AND SAFETY
Polyox: Avoid inhalation of dust and eye contact. Wear eye
protection. If polyox powder enters the eye, immediately
flush with water, continue washing for several minutes, and
obtain medical attention.
Propan-2-ol: HIGHLY FLAMMABLE (keep well away from
naked flames) and IRRITANT/HARMFUL (it is irritating to the
eyes, and the vapour may cause drowsiness or dizziness).
There is no hazard from skin contact with polyox solid or
the solution; if it comes into contact with hands (it feels very
slippery) they should be washed with soap and water.
RESOURCES NEEDED
For the teacher demonstration:
• Approx 100 cm3 of polyox solution in 250 cm3 beaker
• Empty 250 cm3 beaker
• Scissors.
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Activity A2: Food wrappers party tricks
This activity is a teacher demonstration that introduces the idea of thermoplastics compared with thermosets (see also page 11
of this booklet ‘Thermoplastics and thermosets’).
LEARNING OBJECTIVES
Students will:
• recognise that polymers can be divided into two
groups: thermoplastics and thermosets
• explain how thermoplastics can deform (flow) when
heated because of the relatively weak intermolecular
bonds
• explain why thermosets do not change when heated
due to being formed of a molecular network with
much stronger covalent intermolecular bonds.
NOTES
This demonstrates that not all plastics shrivel up when
heated as most students will predict. ‘Moulded’ plastic
packaging (thermoplastics) such as that in a box of
chocolates, around an Easter egg, holding items in place in
a box, etc. has been thermoplastically formed by heating
and stretching. Heating will cause the plastic to revert
to its original form of a flat plastic sheet. Conversely
thermosets do not react like this when heated.
CAUTION: Care should be taken when using a hot air
gun - the gun and the plastics become very hot. Wear eye
protection. If a hair drier or hot air gun is brought from
home it will need to be electrically tested and its use in a
school will not be indemnified by the manufacturer.
Themoplastics:
Take any thermo-formed plastic packaging, put it in a clamp
and heat using a high-power hair drier or hot air gun (paint
stripper). The sheet will not shrivel but will flatten.
Alternatively take a plastic vending cup marked with PS on
the bottom (polystyrene), put it in a clamp and direct the air
from a hair drier or hot air gun into the cup. It will revert to
its original flattened form.
Thermosets:
To demonstrate this, the only way is to show nothing
happens when it is heated. For example, take a polythene
rod, heat it with a hair drier or hot air gun to soften and
bend it by hanging weights on the end. Allow it to cool to
set it – it is a thermoplastic. Do the same with epoxy rod –
nothing happens. It does not soften or bend.
RESOURCES NEEDED
For the teacher demonstration:
• Collection of ‘vacuum formed’ plastic packaging
(sometime called blister packaging)
• Polystyrene cup (not expanded polystyrene)
• Polythene rod
• Epoxy rod
• High-power hair drier or hot air gun
• Clamp stand, bosshead and clamp to hold objects
• Set of slotted masses.
Activity A3: What’s that polymer?
Students look at a number of objects made from polymers that illustrates the range of properties (see also page 9 of this booklet
‘What is a polymer?’).
LEARNING OBJECTIVE
Students will:
• recognise that polymers can have a range of properties
and that these are related to their uses.
RESOURCES NEEDED
NOTES
Polystyrene has a range of properties depending upon the
way it is processed. In Task A students will observe the
properties and how they relate to the uses of the product.
•
•
•
•
•
•
•
The properties of different polymers are related to the
length of the constituent hydrocarbon chain – for example
the range from candle wax to Tupperware to gas mains
pipe. In Task B students will observe how these items differ.
TASK A
Students need access to a range of objects made from polystyrene:
Ceiling tile
Drinking cup – expanded polystyrene
Plastic drinking cup marked PS on bottom
Clear plastic box (e.g. Ferrero Rocher)
Egg carton
Packaging pieces (to pack around fragile items)
Any other item marked with PS on the bottom.
TASK B
Students need access to a range of objects made from
polymers of different chain lengths:
• Candle
• Tupperware container
• Gas mains pipe.
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Activity A4: Making a rubber ball
Students use liquid rubber latex to see how simple cross-linking of liquid polymers can create a solid (see also
page 13 of this booklet ‘How do you make a polymer?’).
LEARNING OBJECTIVES
Students will:
• carry out a procedure to create a solid rubber object
from liquid rubber latex
• interpret the change in terms of cross-linking of
the polymer in liquid form to produce a solid by
the formation of covalent bonds, creating a huge
molecular network
NOTES
Students should wear vinyl gloves when handling wet
rubber latex and making the round ball shape. See the
section on ‘Health and Safety’ for this activity. Note that
the technique used for rolling plasticine into a ball doesn’t
work for the rubber ball – it has to be squeezed. After
forming the ball and dropping into water, it should be
dried using a fabric cloth (paper towels will stick to the
rubber ball).
RESOURCES NEEDED
Each group will need:
• Rubber latex solution
• Lemon juice or vinegar
• Small container (plastic vending cup size)
• Stirrer – ideally wooden or metal, not glass
• Vinyl protective gloves – 1 per student is sufficient
• Eye protection
• Cotton cloth to dry ball
• Access to a large bowl of cold water in which to rinse the
ball (groups of 6 can share).
HEALTH AND SAFETY
Rubber latex solution may cause a skin allergy for
some students, so skin contact should be avoided; eye
protection and vinyl gloves should be worn when handling
it. Once fully cross-linked it is unlikely that there will be an
allergic response, so the solidified rubber can be handled
normally unless a student is particularly sensitive to latex.
The rubber latex solution contains approximately 0.5%
ammonia (similar to the proportion in Copydex), and
care should be taken not to inhale the fumes. In case
of contact with eyes, flush with copious clean water. If
swallowed, drink plenty of water. In both cases immediate
medical attention should be sought to ensure no ill after
effects.
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Innovations In Practical Work: Fantastic Plastic
Activity A5: Testing bounce
Students use balls made of different materials to explore the elastic behaviour of different polymers (see also page
15 of this booklet ‘The properties of rubber’).
LEARNING OBJECTIVES
Students will:
• recognise that natural rubber is an elastomer
• plan a test to measure the elastic behaviour of a rubber
ball
• compare a rubber ball with another ball made from a
less elastic material.
NOTES
In Task A, students will need to use the balls that they have
made in Activity A4. It uses the well-established technique
of dropping the ball from a measured height and noting
the height to which it bounces. (Note that it may be
difficult to control where the ball bounces as it may not
be very round.) There is added interest as students may
compete to see whose ball bounces best!
In Task B, students make a ball from polymorph. This is
moulded by hand after warming in hot water (temperature
about 62 oC). Care has to be taken that students do not
handle the polymorph when it is too hot. See the section
on ‘Health and Safety’ for this activity. Polymorph can be
moulded into many shapes after heating (it doesn’t have
to remain above 62 oC). Ask students to mould it into a
ball of the same size as their rubber ball and test it in the
same way. It will not bounce as readily as the rubber.
In Tasks C and D, students compare the bounce of the
polymorph ball with the rubber ball. Students could go on
to calculate percentage efficiency of bounce at each of the
different drop heights.
RESOURCES NEEDED
Each group will need:
• Rubber ball from Activity A4
• Metre rule
• Clamp stand, bosshead and clamp
• Beaker of hot water (approx. 65 oC)
• Polymorph crystals
• Tongs
• Fabric towel.
HEALTH AND SAFETY
Polymorph should be heated using water in the range 60 –
65 oC, and it is recommended that the water temperature is
monitored with a thermometer. Water at this temperature
can cause scalding, and using water above this temperature
will present an even greater scalding hazard. Polymorph is
very warm when removed from the hot water so care should
be taken when handling. To avoid direct contact with the hot
polymorph and water, it should be dried on a fabric towel
before moulding.
Activity A6: Make your own rubber band
Students make a rubber band from liquid rubber latex to investigate its elastic behaviour (see also page 13 of this
booklet ‘How do you make a polymer?’).
LEARNING OBJECTIVES
Students will:
• carry out a procedure to create a rubber band from
liquid rubber latex
• interpret the change in terms of cross-linking of the
polymer
• plan a test to measure how the extension of the rubber
band varies with the force applied.
NOTES
This is similar to making the rubber ball in Activity A4, but
allows the band to be tested afterwards to demonstrate
Hooke’s Law or simply the concept of elasticity.
The procedure for making the rubber band is relatively
quick, so the beakers of materials could be shared by
several groups.
Students could also remove masses one by one and record
and plot extension for each load back to zero. The activity
can also be extended by testing rubber bands made of
different diameters by using wider supports such as the
neck of glass bottles.
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RESOURCES NEEDED
Each group will need:
• Round pencil (pencils with angular shapes do not coat
well)
• Metre rule
• Clamp stand, boss head and clamp
• Set of slotted masses (10 g).
Each group will need access to:
•
•
•
•
Beaker of rubber latex solution 5 cm deep
Beaker of citric acid solution (lemon juice) 5 cm deep
Beaker of cold water more than 5 cm deep
Large empty beaker to stand pencils in for drying.
HEALTH AND SAFETY
See the notes for Activity A4 for information on how to
handle liquid rubber latex.
Innovations In Practical Work: Fantastic Plastic
Activity A7: Making and testing foam rubber
Students make samples of foam rubber and compare their properties (see also page 15 of this booklet ‘The
properties of rubber’).
LEARNING OBJECTIVES
Students will:
• carry out a procedure to make solid rubber and foam
rubber from liquid rubber latex
• compare the different properties of foam rubber and
solid rubber due to the air pockets in the foam.
NOTES
In this activity students make another kind of object from
liquid rubber latex; the reaction between the citric acid
and sodium bicarbonate in the mixture produces carbon
dioxide which creates foam rubber. This is compared
with a block rubber sheet. As an additional activity,
students could vary the amount of citric acid and sodium
bicarbonate to create sample with different properties.
RESOURCES NEEDED
Each group will need:
•
•
•
•
•
•
Rubber latex solution
Citric acid powder
Sodium bicarbonate powder
Stirrer
2 x small container (plastic vending cup size)
Vinyl protective gloves.
HEALTH AND SAFETY
See the notes for Activity A4 for information on how to
handle liquid rubber latex.
Activity A8: What is a hydrogel?
Students test the properties of a hydrogel (see also page 20 of this booklet ‘Polymer properties related to their uses’).
LEARNING OBJECTIVES
Students will:
• recognise that hydrogels are polymer gels that can
absorb huge quantities of fluid
• find out how much water can be absorbed by a
hydrogel.
RESOURCES NEEDED
Each group will need:
• 2g hydrogel crystals
• 10 cm diameter petri dish
• Dropping pipette
• Access to scales.
NOTES
Students measure a specified quantity of hydrogel into a
dish and then add water until it is no longer possible to
get more water to be absorbed. Weighing the product
and subtracting the initial mass will give the mass of
water absorbed to test the claim that the gel can absorb
its own mass of water. In fact it can absorb several times
its own mass.
Hydrogel is safe to handle but students should wear eye
protection and be asked to wash their hands as a matter
of good practice after handling chemicals.
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Innovations In Practical Work: Fantastic Plastic
Extension X1: Making your own rubber eraser
Students use liquid rubber latex to see how simple cross-linking of liquid polymers can create a solid (see also page 13
of this booklet ‘How do you make a polymer?’).
LEARNING OBJECTIVES
Students will:
• carry out a procedure to create a solid rubber object
from liquid rubber latex
• interpret the change in terms of cross-linking of the
polymer in liquid form to produce a solid by the
formation of covalent bonds, creating a huge molecular
network.
RESOURCES NEEDED
The resources required are the same as those for Activity A4.
HEALTH AND SAFETY
See the notes for Activity A4 for information on how to
handle liquid rubber latex.
NOTES
This activity uses the same procedure as in Activity A4,
but this time the rubber is moulded into a chosen shape
for a pencil eraser. It should be left to cure until it is a
dark yellow and totally dry, and can then be tested by
rubbing out pencil marks.
Extension X2: Why do plastic bags sound different?
Students compare the properties of high density or low density polyethylene, and the activity provides a useful
opportunity for student to devise their own procedures (see also page 20 of this booklet ‘Polymer properties related to
their uses’).
LEARNING OBJECTIVE
Students will:
• recognise some of the differences in properties between
low density and high density polythene.
NOTES
In Task A, students crumple the bags to see differences in
the sound made and separate into ‘crackly sounding’ and
‘quiet’. They then need to measure the density of samples
from each to see if crackly is higher or lower density. There
are two simple approaches that can be used:
• measure the volume and mass of bag directly and
calculate
• measure volume by displacement of water (brings in
some challenges!).
The exercise offers considerable challenge to obtain
accurate results as densities range from 0.92 – 0.98 g/cm3.
Task B is a teacher demonstration of another difference
in properties that density makes. Take a block of material,
use a temperature probe and melt the polymer (until
clear) on a hot plate. The melting point of low density
polythene is 100 oC, whereas for high density it is
130 oC. (CAUTION: Heated polymer will melt.
Care should be taken when applying the tip of the
thermometer, not to touch the melting polymer with
your hands.)
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RESOURCES NEEDED
Each group will need:
• Collection of plastic bags from different companies
• A means of measuring the volume of the bags – by actual
measurement of dimensions or using a displacement can
• Scales.
Innovations In Practical Work: Fantastic Plastic
STUDENT SHEETS
Fantastic plastic
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Innovations
in
Practical Work: Fantastic Plastic
Activity
A1
FANTASTIC PLASTIC:
CAN YOU CUT A LIQUID?
Polymers can be in solid or liquid form. You will observe
a solution of a polymer (called ‘polyox’) to see whether
its properties are the same as other liquids.
Wear eye protection. Avoid
touching the polymer solution.
If you get the solution onto your
hands, wash them thoroughly.
Teacher demonstration
Your teacher will demonstrate an experiment to show
the difference between a polymer solution and other
liquids. The polymer solution is poured from one beaker
to another, and then scissors are used to try to ‘cut’ the
solution.
1. What do you see happen when your teacher pours the
polymer solution?
2. What do you see when your teacher uses the scissors?
3. What happens if you try to cut pouring water?
4. Why do you think you can’t cut pouring water? Use the
idea of a liquid being made of lots of separate molecules
to help your explanation.
5. Why do you think the polymer solution behaves
differently from water when you try to cut it? Use the
idea of long chain molecules to help your explanation.
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scissors
Innovations
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Practical Work: Fantastic Plastic
Activity
A2
FANTASTIC PLASTIC:
FOOD WRAPPERS PARTY TRICKS
The inserts in boxes of chocolates that keep the
chocolates in place are made of polymers. So are the
inserts in other food packaging. Plastic vending cups are
made of polystyrene, another polymer. These materials
show a very unusual behaviour when heated.
Do not touch the hot air gun, the
hot plate or the heated plastic.
Teacher demonstration
Your teacher will demonstrate an experiment to show
what happens when some polymers are heated.
1. What do you see happen when your teacher heats up
the insert from a box of chocolates?
cup
2. What do you see when your teacher heats up a
vending cup?
3. What do you see happen when your teacher heats up
the polyethene rod and the epoxy rod?
4. Some polymers are called thermoplastics. Which
polymers have you seen in this experiment that are
thermoplastics?
5. Polymers that don’t react to heat like thermoplastics are
called thermosets. Why do you think that thermosets
behave differently from thermoplastics when they are
heated? Use the idea of strong bonds between molecules
to help your explanation.
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hair-dryer
Innovations
in
Practical Work: Fantastic Plastic
Activity
FANTASTIC PLASTIC:
WHAT’S THAT POLYMER?
Different polymers have different properties. And
sometimes one polymer can have different properties
depending on how it is processed. It is important that
the right properties are chosen so that the object
performs properly – imagine a cup that melts when you
pour tea or coffee into it!
For this activity you will need to observe the materials in two
different groups of objects.
Task A
Different ways of using polystyrene
1. Look at each object in the first group in turn. Decide
what it has to do and what properties the polymer has
that makes it able to do the job.
2. Make a table that shows the job of the polymer and
which properties are most important when choosing
the polymer – for example melting point, hardness,
brittleness, flexibility, insulation, density, cost, etc.
Task B
Properties and molecular chain length
To compare the properties of the materials, you will need
to make each sample into a cylinder of approximately the
same size.
3. Look at each object in the second group in turn. Decide
how they are different from each other.
4. Try to use the idea of different molecular chain lengths to
explain why these items have different properties.
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A3
Innovations
in
Practical Work: Fantastic Plastic
Activity
A4
FANTASTIC PLASTIC:
MAKING A RUBBER BALL
Rubber is a natural polymer – it is the sap of the
rubber tree and it bleeds out of the tree as a white
liquid when the bark of the tree is cut. This activity
shows you how this simply natural polymer can be
cross-linked to make it solid.
1. Pour 5 cm3 rubber latex solution into small container
such as a plastic cup or a small beaker.
2. Add a squirt of lemon juice or vinegar.
Some people have allergic
reactions to liquid rubber latex.
Wear protective gloves to avoid
touching the liquid rubber,
and wear eye protection. Do
not inhale the fumes, which
contain ammonia.
lemon juice
stirrer
3. Stir to mix.
4. Rubber will solidify and begin to stick to the stirrer.
Add more lemon juice or vinegar if some liquid rubber
remains in the container.
rubber latex
5. Keep stirring until the rubber has completely solidified
and is completely stuck to the stirrer.
6. Drop the solid rubber into the water and squeeze with
your gloved hand to make it into a round ball shape. This
isn’t easy and you have to do this quickly if you want a
round ball.
stirrer
7. Dry the ball using a fabric cloth (not a paper towel).
solid rubber
8. Check that the ball bounces. Keep the ball safe – you
will need it for a later activity.
9. Why do you think that the liquid becomes solid when
you add the weak acid?
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Innovations
in
Practical Work: Fantastic Plastic
Activity
FANTASTIC PLASTIC:
testing bounce
Many solid polymers are bouncy. Here you can test two
different bouncy balls to see if they are the same. You
will start by using the rubber ball you made in activity
A4. Then you will make a ball from a different polymer
called polymorph.
Task A
Testing your rubber ball – how high does it
bounce?
1. Support a metre rule vertically on the floor using a clamp and retort stand.
2. Try dropping the ball and see how easy it is to measure
the height to which it bounces.
drop
height
bounce
height
3. Choose a sensible drop height and use the rule to
measure the height from which you will drop the ball
accurately.
4. Allow the ball to fall then note height to which it
bounces. You may have problems with the balls not
bouncing vertically. You need to think carefully how you
will solve this problem. It will be difficult to measure
the bounce height accurately and you may have to try
several times before you get an accurate measurement.
5. Measure the bounce height accurately 3 times and take
the average.
Task B
Making a ball from a different polymer
(polymorph)
6. Make another ball using ‘polymorph’. To do this, drop
some polymorph beads into warm water (at about
62 oC).
7. The beads will begin to soften and stick together. Lift
them out of the water using tongs, and dry with a
fabric towel.
8. When the polymorph is cool enough to handle, mould it
into a ball making it the same size as your rubber ball.
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The polymorph will be very
warm when it is first taken out
of the hot water. Take care
when you handle it. Do not
handle until it is comfortable
to hold it without feeling your
hands are getting hot.
A5
Innovations
in
Practical Work: Fantastic Plastic
Activity
Fantastic Plastic:
TESTING BOUNCE
Task C
Comparing the two balls
9. Test the bounce of the polymorph ball in the same way
as you tested your rubber ball.
10.Which ball bounces higher?
11.Why do you think one ball bounces higher than the
other? Use the idea of energy transfers to help your
explanation.
Task D
Going further
12.For each ball, change the drop height and measure the
bounce height three times again. Take the average.
13.Repeat this until you have measured the bounce height
for 5 different drop heights.
14.Draw a graph for each ball (on the same axes) to show
the relationship between drop height and bounce
height.
15.How do the two materials compare for bounciness?
Is one material always better than the other for all
drop heights?
drop
height
(cm)
16. Is polymorph a thermoset or a thermoplastic? How can
you tell?
17. Is rubber a thermoset or a thermoplastic? How can
you tell?
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bounce
height
(cm)
A5
Innovations
in
Practical Work: Fantastic Plastic
Activity
A6
FANTASTIC PLASTIC:
MAKE YOUR OWN RUBBER BAND
We take rubber bands for granted. In this activity you
can make your own rubber band from liquid rubber and
also test it for stretchiness.
Task A
Making the rubber band
Some people have allergic
reactions to liquid rubber latex.
Wear protective gloves to avoid
touching the liquid rubber,
and wear eye protection. Do
not inhale the fumes, which
contain ammonia.
1. Dip the blunt end of your pencil into the rubber latex to
a depth of 5 cm (no more!).
pencil
rubber latex
lemon juice
2. Remove your pencil – do not touch the rubber latex film.
3. Dip your pencil coated in rubber into lemon juice so that
the rubber latex is covered.
4. Dip your pencil into cold water so that the rubber latex
is covered.
5. Leave the rubber latex coating to dry for at least
10 minutes.
6. Roll the rubber coating off the pencil carefully forming a
rubber ring – your rubber band. It’s very easy to break the
rubber band as you roll it off your pencil. You may have
to practise this a few times before it works.
7. To make a larger diameter band, use a thicker cylinder,
such as the neck of a glass bottle.
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water
Innovations
in
X1
A6
Practical Work: Fantastic Plastic
Activity
fantastic plastic:
make your own rubber band
Task B
Testing your rubber band
8. Hang the rubber band from a retort stand.
9. Hang masses of a few grams onto the band and
measure the extension.
10.Plot a graph of extension (mm) against weight (N).
11.Does the rubber band stretch uniformly?
12.Does a wide diameter rubber band hold more weight
than a small diameter rubber band?
13.Does a longer rubber band stretch more than a
shorter one?
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Innovations
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Practical Work: Fantastic Plastic
Activity
A7
FANTASTIC PLASTIC:
MAKING AND TESTING FOAM RUBBER
Foam rubber is made by trapping bubbles of gas in
the rubber as the rubber latex cross-links and becomes
solid. Here you will make your own foam rubber and
test it for ‘squashiness’ compared with ordinary rubber.
Some people have allergic
reactions to liquid rubber latex.
Wear protective gloves to avoid
touching the liquid rubber,
and wear eye protection. Do
not inhale the fumes, which
contain ammonia.
Task A
Making your foam rubber
1. Pour 10 cm3 rubber latex solution into a container such
as a small beaker or plastic cup. Make sure the container
has a small diameter base (less than 5 cm).
2. Add 10 cm3 water to the rubber latex solution.
container 1
water
rubber latex
3. In another container put 1 spatula of sodium bicarbonate
powder.
container 2
4. Add 1 spatula of powdered citric acid to the sodium
bicarbonate powder and mix well.
citric
acid
sodium
bicarbonate
5. Pour the sodium bicarbonate powder/citric acid powder
mix into rubber latex solution and stir quickly to mix
then leave it to foam.
6. When it has finished foaming, drop the foam rubber
cushion into water to wash it.
7. Leave the foam rubber cushion for 30 minutes to dry
and harden before using it.
container 2
container 1
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stirrer
Innovations
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X1
A7
Practical Work: Fantastic Plastic
Activity
fantastic plastic:
making and testing foam rubber
Task B
Making a block rubber sheet
container 1
container 2
8. Pour 10 cm3 rubber into the 5 cm diameter base
container.
9. Mix 1 spatula citric acid powder with 10 cm3 water to
make citric acid solution.
rubber
citric
acid
10. Pour the citric acid solution into the rubber latex
solution and stir to mix. The rubber latex will cross-link
and form a solid in the bottom of the container.
11. Wash the solid rubber in water and leave for 30 minutes
to dry thoroughly before testing.
container 2
stirrer
container 1
Task C
Comparing your rubber samples
12. Measure the thickness of the two samples.
13. Place a 100 g mass on the top of each sample and
measure the thickness of the sample by measuring the
distance between the bench and the 100 g mass.
14. Increase the mass and measure the thickness of the
sample each time.
15. How much force does it require to squash the foam
rubber to the same thickness as the solid rubber?
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100g mass
foam
rubber
100g mass
block
rubber
Innovations
in
Practical Work: Fantastic Plastic
Activity
FANTASTIC PLASTIC:
WHAT IS A HYDROGEL?
Hydrogels are special polymers that are supposed to
be able to absorb more than their own mass of liquid.
Here you will test if this is true and just how much they
can absorb.
1. Weigh 2 g hydrogel in a large petri dish.
2. Add water gently using a pipette, making sure it is totally
absorbed by the polymer.
3. As you get to the limit of the polymer’s absorption, you
will find it takes longer and longer for the water to be
absorbed.
4. When you are sure you are at the limit of the hydrogel’s
absorption, weigh the petri dish and its contents to work
out how much water was absorbed.
5. Is it true that the hydrogel can absorb more than its own
mass of water?
These hydrogels are used in disposable baby’s nappies to
keep the baby dry. There are also hydrogels designed to
absorb oil when an oil tanker crashes and spills its oil into the
sea. And there are hydrogels that absorb toxic chemicals that
can be spilt onto the road if there’s an accident with a truck
carrying chemicals on the motorway.
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Wear eye protection. The
hydrogel is safe to handle
but wash your hands after
handling and before eating.
A8
Innovations
in
Practical Work: Fantastic Plastic
Extension
X1
Fantastic Plastic:
MAKING YOUR OWN RUBBER ERASER
We use rubber erasers all the time but don’t stop to
think how they are made or how they work. Here you
will make your own eraser and test it.
1. Put 10 cm3 rubber latex into a cup.
2. Add a squirt of lemon juice and stir.
3. Drop the solid rubber into clean water and mould into
the shape you want for your eraser.
4. Dry using a towel.
5. Allow to dry thoroughly by leaving it for at least 30
minutes in a warm place. It may be a good idea to
leave it to dry overnight. You will know that it is
thoroughly dry as it will appear a much darker yellow
than when it is newly made.
6. Test your new eraser by making some pencil marks on
paper and trying to rub them out.
7. What to you notice about the way that an eraser
removes the pencil marks? A pencil mark is a line of
graphite sticking to the paper. Where does it go when
you rub it out?
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Some people have allergic
reactions to liquid rubber latex.
Wear protective gloves to avoid
touching the liquid rubber,
and wear eye protection. Do
not inhale the fumes, which
contain ammonia.
Innovations
in
Practical Work: Fantastic Plastic
Extension
X2
FANTASTIC PLASTIC:
WHY DO PLASTIC BAGS SOUND DIFFERENT?
We use thousands of plastic bags each day. Most of
them are made of a polymer called polyethylene.
Here you will find out why some plastic bags seem
different from others, even though they are made from
the same material.
Task A
Sorting out the plastic bags
1. Take a range of plastic bags from different companies.
Crumple them to see the different sounds they make.
Separate them into two groups – the ‘crackly’ ones and
the ‘quiet’ ones.
2. Some bags are made out of high density polyethylene.
Others are made out of low density polyethylene. High
density polyethylene has very different properties from
low density polyethylene.
3. Work out how to measure the density of two very
different plastic bags to see if high density polyethylene
makes bags that are crackly or quiet.
4. How can you be sure your method was accurate? What
can you do to make it more accurate?
Task B
Teacher demonstration: melting polythene
5. Your teacher will use a high density polyethylene sample
and a low density polyethylene sample, heat each one up
on a hot plate and take the temperature when it melts.
6. Which one do you think will melt at the lower
temperature?
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Do not get close to the hot plate to
avoid burns to the skin.
Innovations In Practical Work: Fantastic Plastic
BACKGROUND SCIENCE
Fantastic plastic
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Innovations In Practical Work: Fantastic Plastic
BACKGROUND SCIENCE
This section gives further information about the nature of polymers, their molecular structure and macroscopic
properties, and their future development and applications.
NATURE OF POLYMERS
Chemically, the building blocks, or monomers, from which polymers are made can be extremely simple, and possibly
the most simple of all is polyethylene, where the repeat unit in the idealised chain is just -CH2-CH2-. Since all the
repeat units are the same, such a material is termed a homopolymer.
Most synthetic polymers are relatively simple molecules. Indeed, many important materials are simple variants on
the polyethylene structure and can be thought of simply as -CH2-CHR- ; that is, where one of the hydrogen atoms
has been replaced by a chemical group designated R. When R = CH3 then the resulting polymer is polypropylene,
currently the second most important bulk commodity polymer after polyethylene.
Copolymerisation is a widely used method for producing materials with desirable properties from existing
monomers. If a molecule contains two repeating units, A and B, with the molecular backbone consisting of a simple
random sequence of A and B units, then the resulting polymer will be a random copolymer.
Alternatively, the A and B units may be arranged in blocks; a block of A units followed by a block of B units would
be termed a diblock copolymer while a block of A followed by a block of B followed by a block of A is a triblock
copolymer. Both of these molecular architectures would be linear molecules.
Another structure is where short A blocks are attached to a long backbone of B units to give a graft copolymer;
in this case, the molecule would be a highly branched structure. Finally, the A and B units could alternate along
each molecule to give an alternating copolymer. Such a structure can also be thought of a homopolymer in which
the repeat unit takes the form AB. Although this may appear as mere semantics, many technologically important
materials are prepared by reacting together two different chemicals in exactly this way to produce long chain
molecules. Poly(ethylene terephthalate) (PET) is produced by a reaction between ethylene glycol and terephthalic acid
to produce a polymer which is widely used in packaging applications, such as drinks bottles, and as fibres.
Synthesis
The preceding section has described a range of different molecular forms as being polymeric but what is it that they
all have in common? PET constitutes a good example of the concept of functionality: the ethylene glycol molecule
contains an hydroxyl group at each end, both of which are capable of reacting; the terephthalic acid molecule
contains a carboxylic acid group at each end, both of which are, again, capable of reacting. Thus, when a molecule
of ethylene glycol reacts with a molecule of terephthalic acid, the product still contains reactive sites at both ends
and can go on to react further. In this case, each constituent contains two reactive sites or functional groups, such
that they are termed bifunctional.
48
ethylene glycol
Gatsby Science Enhancement Programme
terephthalic acid
Innovations In Practical Work: Fantastic Plastic
If such reactions were to continue the result would be a long linear structure. If, however, we were to introduce just
a few addition species that contained three rather than two functional groups then the result would be branched,
rather than purely linear molecules. As we increase the average functionality of the monomers, so the tendency
for branching increases until at some point the result will be a network. An example of this type of reaction, which
involves many polyfunctional monomers, is the curing reaction of an epoxy resin – once the network has formed the
initially liquid resin becomes solid.
How many is ‘many’?
This question is difficult since there is no absolute answer. Rather, it depends on what property or characteristic is
of interest. For example, one of the important characteristics that defines the behaviour of a molten polymer is the
tendency for molecules to become entangled with one another, a phenomenon that dramatically affects the flow
characteristics, or mechanical properties of the melt. Below a critical point, termed the entanglement molecular
weight, the viscosity of the material only increases slowly as the molecules get longer; above this critical threshold,
the viscosity increases dramatically as the system becomes increasingly entangled as the molecules get longer.
The most important ‘bulk commodity’ polymers – polyethylene and polypropylene – are both materials that include
a significant crystalline fraction (40-90%). These crystals contain molecules which, in the ideal case, repeatedly
fold back upon themselves. This is characteristic of polymers and, therefore, one can argue this tendency to fold
defines a polymer; in polyethylene, the onset of chainfolding occurs for chains containing more than about 150
carbon atoms. Systems which do not exhibit polymeric characteristics such as those examples described above are
commonly termed oligomers.
Condensation reactions and addition reactions
Condensation or, more correctly, step-growth polymerisation reactions, are characterized by the random reaction
of functional groups within the system. Such reactions are exploited in the formation of, for example, polyamides
(e.g. Nylon), which are produced by reacting equal molar quantities of a diamine and a dicarboxylic acid, that is two
bifunctional monomers. This can be represented:
AA + BB => AABB etc.
where A and B represent the functional groups on each of the molecules. Initially all the molecules in the system
will either be AA or BB but, after the first reaction occurs, there will, additionally, be just one AABB molecule. The
probability will therefore be that the next reaction will be as shown above. In other words, it will probably not
involve AABB. Thus this type of process does not tend to result in the early production of long molecules. After the
reaction is complete there will be a distribution of molecules that vary statistically with respect to their length. As
discussed above, polymeric characteristics are strongly dependent on molecular length. Will the resulting material
behave like a polymer? This will depend on the precise nature of the length distribution we have produced.
Addition reactions involve three distinct steps and, as such, differ fundamentally from the process described above
in which all functional groups are equivalent. Addition reactions progress as additional monomers attach themselves
to the existing polymer chain. The initial creation of polyethylene (an addition polymer) at ICI on 27th March
1933 was followed by much activity attempting to reproduce this feat, but with limited success. The key to the
process was the inclusion of the correct amount of oxygen in the reaction vessel: at the high pressures and high
temperatures initially used, oxygen serves as an initiator for a rapid chain reaction between ethylene monomers
involving free radicals (i.e. a neutral species containing unpaired electrons – effectively, half of a covalent bond). This
continues until the radical is destroyed or transferred to some other molecule.
Initiation
Propagation
Termination
R· + A => RA·
RA· + A => RAA·
R(A)nA· + R’· => R(A)nAR’
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Because each propagation step occurs rapidly, this process results in the rapid growth of long molecules, but the
termination process is still a random event and, therefore, the result is again a material that contains a range of
molecular lengths.
Attempts to control the polymerisation process more closely resulted in the development of alternative approaches.
In the 1950s Ziegler, Natta and others developed catalysts that enabled many other polymer types to be produced.
In the late 1980s so called metallocene catalysts were developed which offer far greater control of molecular
architectures than was possible previously. The full implication and possibilities of this are, as yet, not fully explored.
Variations in structure
Polymer molecules are structurally complex beasts. In general, a sample of polymer will contain a variety of
molecular forms which may differ from one another in terms of:
•
•
•
•
•
molecular length
number of branches or side groups
length of branches
distance between branches or side groups
average number of branches or side groups per molecule.
All of these can apply even to one single material – say, polyethylene. Therefore it is critically important when
thinking of polyethylene that this variability is remembered. Polyethylene A and polyethylene B may both be
labelled polyethylene, in the same way that both Edam and Gorgonzola will both be found under ‘cheese’ in the
supermarket. Edam and Gorgonzola are very different; polyethylene A and polyethylene B may also be!
MOLECULAR STRUCTURE AND MACROSCOPIC PROPERTIES
The macroscopic properties of engineering plastics depend upon many factors. However, of these, the most
important are:
• the physical and chemical properties of the constituent monomers
• the way in which the monomers are joined together.
Monomer chemistry
Many of the fundamental characteristics of a material are determined at the monomer level. For example poly(vinyl
acetate) is soluble in water because the acetate groups give considerable scope for hydrogen bonding. Polyacetylene
is produced by the polymerisation of CH≡CH and has a molecular structure that, in one dimension, can be
represented:
-CH=CH-CH=CH-CH=CH-
That is, the molecular backbone contains alternating carbon-carbon double and single bonds. However, in practice,
the extra electrons in the double bonds are not confined as shown in the above diagram but rather, are delocalised
along the complete chain. Consequently, this material is a good conductor of electricity and, when suitably
processed, can exhibit electrical conductivities comparable with the best metallic conductors, such as copper and
silver. Conversely, the polyethylene backbone is composed of a simple sequence of carbon-carbon single bonds and,
therefore, does not have the extra electrons in the backbone to render it conducting; these electrons are involved
instead in bonding with the extra hydrogen atoms.
50
-CH2-CH2-CH2-CH2-CH2-CH2-
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Molecular architecture
The properties of polymers are not just determined by the monomer units; properties can be ‘engineered in’ by
controlling the molecular architecture. For example, an idealised polyethylene molecule is a linear sequence of
ethylene units and constitutes a highly regular structure that crystallizes readily and exhibits a melting temperature
of the order of 130 oC. When in crystalline form, the molecules are packed together more densely than in less
ordered structures and therefore have a slightly higher density. Such materials are therefore referred to as high
density polyethylene (HDPE). In practice, a few branches are in fact deliberately introduced into HDPE since this
has been found to result in better overall properties. Introducing more and more branches ultimately results in a
copolymer that is termed linear low density polyethylene (LLDPE). These materials contain more branches and are
therefore less crystalline than HDPE: they soften at a lower temperature, and exhibit lower mechanical moduli and
reduced tensile strength. However, LLDPEs have greatly improved resistance to environmental stress cracking – e.g.
utensils made from LLDPEs are ‘dishwasher proof’.
Cross-linking
Historically, cross-linking constitutes one of the first means by which the properties of a polymer were modified to
make it more suitable for use in a particular application (Goodyear’s vulcanised natural rubber). Cross-linking is now
widely used to produce artefacts from many thermosetting setting systems. As the density of cross-links increases so
the material becomes increasingly stiff and, therefore, by controlling the cross-link density it is possible to produce
materials of the desired stiffness.
The characteristic bounce of rubber results from the deformation of a network that springs back when the force is
released: this occurs because all the deformed molecular conformations between the cross-links or nodes relax back
to their equilibrium conformations. These elastomeric properties stem from the presence of a network, and such
a network would indicate a thermoset – the term ‘thermoplastic elastomer’ therefore appears to be an oxymoron.
However, by clever control of the molecular architecture of block copolymers, it is possible to achieve just this effect.
Consider, for example, a triblock copolymer in which the two terminal blocks are composed of monomers of a
material that, at the temperature at which the product will be used, is a tough crystalline solid. Now, if the central
block is composed of a monomer which confers soft elastic properties then we will be able to form our object
by heating the block copolymer to a temperature at which the end blocks melt and then cooling it down. If the
monomers in the terminal and mid-blocks are thermodynamically immiscible, then we can form a wide variety of
different structures by varying their relative length but, under all conditions, the end-blocks will crystallize with other
end-blocks leaving the stretchy mid-blocks grouped together in between.
Thus, we have replaced our molecular nodes in a conventional elastomer with crystalline aggregates of a different
chemical composition. The overall properties of this material will not only be a reflection of the elastic mid-block, it
will also reflect the presence of the tough end-blocks. Why would this be useful? Consider the sole of a sports shoe
- it needs to be flexible (provided by the mid-block) but also durable (provided by the end-blocks).
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THE FUTURE
While the vast bulk of polymers produced today are electrical insulators, some polymers conduct electricity very
well indeed. These materials are generally referred to as conducting polymers. The process by which this happens
has much in common with conventional semiconductors in that it requires the introduction of dopant ions. Silicon
can equally well be doped to make it n-type (conduction primarily by electrons) or p-type (conduction primarily by
holes), though a given conducting polymer tends to conduct either by electrons or by holes.
All the possibilities of current microelectronics can, theoretically, be replicated using combinations of electron
transport polymers and hole transport polymers, except that silicon does it all so much better. In which case, why
bother with polymers at all? Single crystal silicon is expensive to produce, inflexible and can only be produced in
small sizes. Polymers are cheap to produce, flexible and can be processed in whatever size you want. For example,
a ‘use-once-then-throw-away’ RFID (radio frequency identification) tag to replace the bar code on an item of
shopping doesn’t need the processing power of a PC microprocessor, but it does need to be cheap.
By creating a suitable junction between a hole-conducting polymer and an electron-conducting polymer, and
applying a voltage across the device, charges will move in opposite directions until they meet at the junction
where they combine and give out light. This is the basis of a polymeric organic light-emitting diode (OLED) and
the potential applications for such a device are enormous. Again, this can all be done at present using inorganic
semicondustors so why bother? Current LED traffic lights are certainly bright, but this is achieved by bringing
together lots and lots of individual LEDs, because you cannot cheaply produce large area inorganic LEDs. Since many
conducting polymers are soluble, you could just ‘paint’ them where you need them, easily producing large-area
devices. Getting a good colour balance is not straightforward though, and research on this continues.
For display applications, OLEDs have considerable benefits over existing LCDs in that they can be viewed from
almost any angle, have improved clarity and brightness and, whereas conventional LCDs require a backlight and
effectively act as a shutter, OLEDs generate light only when and where required. Although devices containing OLED
displays are already available, in reality these constitute a hybrid technology in that, while the active light-emitting
layer is organic, the drive circuitry that controls the display is based on conventional rigid inorganic materials. The
real benefits of this technology will therefore come when organic display elements are combined with organic
drive circuitry to produce fully flexible devices. Although the concept has already been proven by major electronics
companies, such as Sony, at present the number of pixels is limited.
If it is possible to create lighting from electricity then what about the other way round? While inorganic
semiconductor photovoltaic cells are quite efficient, again, large areas are required and this is economically
unattractive. Polymer-based photovoltaic cells have been produced but, currently, the efficiency is very low.
Nevertheless, a lot of work is going on to solve this problem, and approaches such as the inclusion of fullerene
derivatives are showing promise. The possibility of cheap photovoltaic cells capable of efficiently using energy from
the Sun could have significant implications.
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Innovations In Practical Work: Fantastic Plastic
REFERENCES AND FURTHER READING
Websites
Additional resources can be found on the SEP website (see page 54). Links to other particularly relevant websites are also listed
on the SEP website. Some examples are given below.
Royal Society of Chemistry (www.rsc.org)
There is a great deal of useful and accessible information on this site, including experimental instructions on making nylon.
Practical Chemistry (www.practicalchemistry.org)
This website contains a growing collection of chemistry experiments, including a useful section on polymers.
Wikipedia (http://en.wikipedia.org)
The section ‘Polymer’ provides a reliable explanation of polymer properties and history with useful links.
Science is fun in the lab of Shakhashiri (http://scifun.chem.wisc.ed/)
This website of the University of Wisconsin-Madison has a ‘Chemical of the Week’ – the section on polymers gives a good
explanation of the various common polymers with details of recycling.
BBC Bitesize (http://www.bbc.co.uk/schools/gcsebitesize/)
The section on useful products from oil is relatively low level but is good for basic information.
Books
Go with the flow: investigating bouncy fluids and other strange materials
Hollamby P (2005) Publisher: Gatsby Science Enhancement Programme (ISBN 978-0750641326).
This booklet includes information on some interesting and unusual polymer materials, including details of how to make and
test slime.
Introduction to polymers
Young R J and Lovell P (1991) Publisher: International Thomson Computer Press (ISBN 978-0748757404).
A good overall textbook that covers the underlying physical principles of polymeric materials.
Polymer physics
Gedde U (1995) Publisher: Kluwer Academic Publishers (ISBN 978-0412626401).
Also a good textbook for the underlying physical principles.
Polymers: Chemistry and Physics of Modern Material
Cowie J M G and Arrighi V (2007) Publisher: CRC Press Inc. (ISBN 978-0849398131).
Similar to the two above but a general textbook with a more chemical emphasis.
Plastics materials
Brydson J A (1999) Publisher: Butterworth-Heinemann (ISBN 978-0750641326).
Very different from those listed above – focuses instead on different classes of polymers so has more of an engineering feel to it.
An introduction to the mechanical properties of solid polymers
Ward I M and Sweeney J (2004) Publisher: John Wiley & Sons (ISBN 978-0471496267).
A good introduction that discusses the mechanical aspects of polymers.
SOURCES
The following photographs have been reproduced from BigStockPhoto.com: widescreen LCD monitor on page 1 (© Robyn
Mackenzie); plastic medicine bottles on front cover and on page 1 (© Paula Wolf Borek); DNA structure on page 1 (© Pawel
Szczesny); misty forest on page 5 (© Mikhail Lavrenov); cotton plants on page 5 (© Deborah Howe); vintage bicycle on page 6
(© Peter Baxter); Bakelite camera on page 6 (© Dariusz Gudowicz); polythene milk bottle on page 7 (© Colin and Linda McKie);
polystyrene fast-food container on front cover and on page 7 (© Nguyen Thai); PVC pipe on page 7 (© Simon Krzic); nylon rope
on page7 (© Freddie Thompson).
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Innovations In Practical Work: Fantastic Plastic
DOWNLOADING RESOURCES FROM THE SEP WEBSITE
SEP produces a range of digital resources to accompany each of its
publications. These are available from the ‘SEP Associates’ area of
the website (www.sep.org.uk).
Membership of SEP Associates is free to all science teachers and
technicians in UK schools. To join, fill in the online form on the
website. SEP is wholly funded by the Gatsby Charitable Foundation,
and joining SEP Associates entitles its members to additional
benefits, including offers of free publications and other resources.
www.sep.org.uk
I
P
W
:F
P
Activity
A1
I
FANTASTIC PLASTIC:
CAN YOU CUT A LIQUID
P
W
F
Activity
P
A1
FANTASTIC PLASTIC:
CAN YOU CUT A LIQUID?
Polymers can be in solid or liquid form. You will observe a
solution of a polymer (called ‘polyox’) to see whether its
properties are the same as other liquids.
Wear eye protection. Avoid
touching the polymer solution.
If you get the solution onto your
hands, wash them thoroughly.
Polymers can be in solid or liquid form. You will
observe a solution of a polymer (called ‘polyox’) to
see whether its properties are the same as other
liquids.
Teacher demonstration
Teacher demonstration
Your teacher will demonstrate an experiment to show the
difference between a polymer solution and other liquids. The
polymer solution is poured from one beaker to another, and then
scissors are used to try to ‘cut’ the solution.
1. What do you see happen when your teacher pours the
polymer solution?
Your teacher will demonstrate an experiment to show the
difference between a polymer solution and other liquids. The
polymer solution is poured from one beaker to another, and
then scissors are used to try to ‘cut’ the solution.
scissors
Wear eye protection. Avoid
touching the polymer solution. If
you get the solution onto your
hands, wash them thoroughly.
2. What do you see when your teacher uses the scissors?
3. What happens if you try to cut pouring water?
1. What do you see happen when your teacher pours the
polymer solution?
4. Why do you think you can’t cut pouring water? Use the idea
of a liquid being made of lots of separate molecules to help
your explanation.
2. What do you see when your teacher uses the scissors?
5. Why do you think the polymer solution behaves differently
from water when you try to cut it? Use the idea of long chain
molecules to help your explanation.
3. What happens if you try to cut pouring water?
4. Why do you think you can’t cut pouring water? Use the idea of
a liquid being made of lots of separate molecules to help your
explanation.
5. Why do you think the polymer solution behaves differently
from water when you try to cut it? Use the idea of long chain
molecules to help your explanation.
1 of 1
These materials may be edited for use
in schools and colleges provided the
original copyright is acknowledged.
34
35
1 of 1
PDF file of booklet
PDF files of student sheets
Word files of student sheets
On the SEP website there is a
In addition to the whole booklet,
Each student sheet also exists as a
downloadable PDF file of the
the student activities are available
Word file. These files can be edited
whole booklet that can be printed
as separate PDF files.
in order to adapt the activities to suit
individual circumstances.
or viewed on screen.
PowerPoint presentations
Links to other sites
These contain the photographs and
The SEP website provides links
drawings shown in this booklet.
to other useful websites on
They can be used to create custom-
this topic.
made presentations.
54
© 2008 Gatsby Science Enhancement Programme
Gatsby Science Enhancement Programme
Innovations In Practical Work: Fantastic Plastic
OBTAINING THE PRACTICAL RESOURCES
SEP works in close partnership with Middlesex University Teaching Resources (MUTR) in
identifying and developing low-cost practical resources to accompany its publications. Many
other resources for school science and technology can also be purchased from MUTR.
The latest prices of the products shown below and further information can be obtained
from www.mutr.co.uk, or by requesting their latest catalogue. Orders can be made by post,
telephone, fax or email by providing an official school order number or by credit card.
Polyox
233-004
A solution of polyox
(polyethylene oxide) behaves as
an ‘elastic liquid’, i.e. it ‘selfsyphons’.
Rubber latex
211-019
Adding a weak acid to liquid
rubber latex creates crosslinks between the molecules,
producing solid rubber.
Polymorph
PL1 004A
A tough ‘engineering’ material,
but which can be softened and
moulded at a relatively low
temperature.
Hydrogel
233-003A
This white power can absorb
many times it own weight in
water producing a gel.
Shape memory polymer
234-100
When heated, shaped and
cooled it will retain its new
shape; when heated again it will
revert to a flat sheet.
Slime
SEP 002
This pack contains the ingredients
for making green slime (PVA, borax
and fluorescein).
Smart putty
SEP 011
When pulled slowly smart putty
will stretch, but when pulled
quickly it will snap.
Smart gum
SEP 012
Smart gum is similar to smart
putty, but has a lower viscosity
and flows more easily.
There are many other novel and
interesting polymer materials
available from MUTR, such
as magic snow and liquid
polymorph, as well as a range
of conventional plastics such
as polypropylene, acrylic and
polystyrene sheets.
Gatsby Science Enhancement Programme
55
You can download the written materials in this booklet, and find further
information from: Science Enhancement Programme www.sep.org.uk
The Science Enhancement Programme (SEP) is part of Gatsby Technical Education Projects.
It undertakes a range of activities concerned with the development of curriculum resources
and with teacher education.
Gatsby Science Enhancement Programme
Middlesex University
S37 Stable Yard
Bramley Road
London N14 4YZ
Email: info@sep.org.uk
Web: www.sep.org.uk
You can order a range of plastics materials and other practical resources from
Middlesex University Teaching Resources.
Teaching Resources
Middlesex University
Unit 10, The IO Centre
Lea Road, Waltham Cross
Hertfordshire EN9 1AS
Tel: 01992 716052
Fax: 01992 719474
Email: sales@muventures.co.uk
Web: www.mutr.co.uk
www.sep.org.uk
Published by the Science Enhancement Programme
Designed and Produced by: Pluma Design Ltd.